The present invention relates generally to systems and methods for controlling an electric motor and, more particularly, to a controller and associated drive assembly for controlling a brushless direct current (BLDC) motor capable of power sharing, time sliced operation.
Electrical machines are used throughout a great number of devices today, and typically consist of motors, which convert electrical energy into mechanical energy, and generators, which convert mechanical energy into electrical energy. Generally, electrical machines fall into one of three categories: polyphase synchronous machines, polyphase asynchronous (i.e., induction) machines and direct current (DC) machines. Typical machines consist of two main portions: a stationary, outside portion called a stator, and a rotating, inner portion called a rotor. The rotor of typical machines is mounted on a stiff rod, or shaft, that is supported in bearings so that the rotor is free to turn within the stator to produce mechanical energy.
In one type of synchronous machine, a permanent magnet, brushless direct current (BLDC) machine, the stator is composed of windings that are connected to a controller, and the rotor is composed of two or more permanent magnets of opposed magnetic polarity. The controller includes a driver that generates poly-phase alternating input currents to the stator windings. One conventional driver includes a series of Insulated Gate Bipolar Transistors (IGBT""s) electrically connected to the phase windings of a BLDC motor. For example, for-a three-phase BLDC motor, a conventional driver includes six IGBT""s arranged in three half-bridges, where each half-bridge generates a drive for one phase of the motor.
As the rotor rotates within the stator, and the magnets of one polarity approach cores of the stator about which the windings are wound, and that conduct the opposed polarity, sensors signal the angular position of the rotor to the controller which, in turn, controls the alternating currents to switch the polarity of the magnetic field produced by windings of the stator. For example, a three-phase BLDC motor can have two, four or more permanent magnets with alternating magnetic polarities mounted on its rotor. The required rotating magnetic field is produced by current through the stator windings. And the three phases of the current are switched in sequence, which is dictated by the angular position of the rotor.
In many BLDC motor systems, the speed of the BLDC motor is controlled by the driver pulse modulating, such as pulse width modulating, the input voltage generated by the controller. By pulse-width-modulation (PWM) of the input voltage, the driver and, thus, the controller controls the average input currents to the windings by using xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d states. During the time the input currents through the windings are increasing, the voltage supply provides constant voltage to the driver at a level at least as high as the motor voltage required for the desired speed of operation. Once the currents have reached the required levels for the desired speed of the motor, the duty ratio is changed to that required to maintain the currents at or near the required level of current.
Conventional BLDC motor systems that include a driver comprising a series of IGBT""s are adequate in controlling the speed of BLDC motors at low frequencies and currents. A standard driver including six IGBT""s can drive a three-phase motor (two IGBT""s per phase) with a switching frequency up to approximately 20 kHz if the maximum current of approximately 50 Amps is not required for more than a few minutes. In this regard, each IGBT can typically operate with a maximum switching frequency of approximately 20 kHz at a maximum of 50 Amps. Whereas such drivers can control the speed of BLDC motors at low frequencies, such drivers that drive higher power (e.g., greater than one horsepower) and higher voltage (e.g., greater than 200 volts) three-phase motors cannot typically switch at a frequency higher than 20 kHz when the driver comprises IGBT""s. The limit in switching frequency is due to the losses associated with switching the IGBT""s and the average current being switched. What makes the IGBT poor at higher frequencies is that the gate of the transistor is not directly connected to the gate drive circuit (hence insulated gate) and, thus, the electrical charge cannot be quickly removed. The rate at which the electrical charge can be applied or removed fixes the time the IGBT is transitioning between its xe2x80x9coffxe2x80x9d and saturated xe2x80x9conxe2x80x9d states. As the switching frequency increases, the percentage of time that the IGBT is in these transitional regions increases. Also, as current is flowing while the IGBT transitions between states, the power dissipated while the IGBT is in these transitional regions increases. And while other, more advanced products are available that can run at higher frequencies, such products are factors of 50 times more expensive than conventional IGBT""s and are not production items.
In light of the foregoing, various embodiments of the present invention provide an improved controller and associated drive assembly for power sharing, time sliced control of a brushless direct current (BLDC) motor, where the motor includes a predetermined number of phase windings. Various embodiments of the controller and associated drive assembly of the present invention include a plurality of switching elements, such as IGBT""s, arranged such that the power dissipation of each switching element is reduced, as compared to the switching elements of a conventional driver. In this regard, the switching elements can be operated at a desired frequency with a duty cycle less than the duty cycle of the operating frequency at which the switching elements of the drive assembly collectively drive the BLDC motor. Further, even though the drive assembly of embodiments of the present invention provide for more switching elements than the conventional driver, the number of transitions of the switching elements does not increase. Thus, operation of the drive assembly of the present invention does not reduce the efficiency of the switching elements, as compared to a conventional drive assembly.
According to one embodiment, a controller for controlling a BLDC motor controller includes a drive assembly, and a processing element. The processing element is in electrical communication with the drive assembly and the BLDC motor, and the processing element is capable of controlling operation of the drive assembly. The drive assembly, on the other hand, is in electrical communication with a power supply and the BLDC motor. As such, the drive assembly is capable of receiving a voltage output of the power supply and is capable of providing a pulse-width-modulated input voltage to the BLDC motor.
The drive assembly includes a plurality of half-bridge assemblies that each include two switching elements, such as insulated gate bipolar transistors (IGBT""s). In this regard, at least two half-bridge assemblies are electrically connected to each phase winding of the BLDC motor. And each switching element is capable of operating at an operating frequency with a first duty ratio such that the half-bridge assemblies are capable of providing the pulse-width-modulated input voltage to the respective phase winding of the BLDC motor at the operating frequency with a second duty ratio higher than the first duty ratio. More particularly, the half-bridge assemblies that are electrically connected to each phase winding of the BLDC motor are capable of providing the pulse-width-modulated input voltage to the respective phase winding of the BLDC motor at the operating frequency with a second duty ratio equal to the product of the number of half-bridge assemblies electrically connected to the respective phase winding and the first duty ratio.
According to another embodiment, the drive assembly comprises a plurality of drive elements that each include the predefined number of half-bridge assemblies. In this embodiment, each half-bridge assembly of each drive element is electrically connected to a respective phase winding of the BLDC motor. For example, the BLDC motor can include a first, a second and a third phase winding, where the drive assembly comprises a plurality of drive elements that each comprise a first half-bridge assembly, a second half-bridge assembly and a third half-bridge assembly. In this regard, the first half-bridge assemblies can be electrically connected to the first phase winding, the second half-bridge assemblies can be electrically connected to the second phase winding, and the third half-bridge assemblies can be electrically connected to the third phase winding.
According to yet another embodiment where the drive assembly comprises a predefined number of drive elements that each comprise a plurality of half-bridge assemblies, each half-bridge assembly of the drive elements is electrically connected to a respective phase winding of the BLDC motor. For example, the BLDC motor can include the first, second and third phase winding. In this regard, the drive assembly can comprise a first, a second and a third drive element that each comprise a plurality of half-bridge assemblies. Thus, the half-bridge assemblies of the first drive element can be electrically connected to the first phase winding, the half-bridge assemblies of the second drive element can be electrically connected to the second phase winding, and the half-bridge assemblies of the third drive element can be electrically connected to the third phase winding.